1. Field of the Invention
The present invention relates to a field assembly of an electric motor incorporated in power-driven tools, and more specifically to a coil terminating structure of a field assembly.
2. Description of the Related Art
In a conventional coil terminating structure, a coil winding 20 is wound on a field assembly 23, which includes a core 21 and a terminal plate 22 mounted on one end of the core 21 as shown in FIG. 4. A starting end 20a and a terminal end 20b of the coil winding 20 are respectively received by terminal structures, each including a terminal well 24 integrally formed on the terminal plate 22 and a terminal 25 inserted in the terminal well 24. The coil winding 20 is wound on the core 21 by a coil winding machine. Each end 20a or 20b of the coil winding 20 is held in each terminal well 24 under tension. When a commutator motor with such a field assembly is incorporated in power-driven tools such as electric hammers or drills causing vibration or shock, both ends of the coil winding are exposed to concentrated stress. Especially the starting end of the coil winding is pressed by the primary portion of the coil winding and has only a small degree of freedom, thus causing undesirable breaking or short circuit. In order to solve such problems, improved structures of a field assembly have been proposed, which includes a structure wherein a coil terminating portion of the terminal well is fixed by an adhesive (Japanese Patent Laying-Open Gazette No. 62-88435) and another structure which has a slack between the terminal well and the coil wire (Japanese Patent Laying-Open Gazette No. 2-214433).
However, the former structure prevents broken wires only at coil terminating portions and can not prevent the same on the boundary between the coil wire and the adhesive or a contact between the coil wire and another portion. The latter structure can not ensure a sufficient amount of slack and may cause broken wires when large vibration or shock is applied to.